While vegetating, the quantity of water or the humidity ratio of the tree varies in accordance with the species, the locale and the season. Also, there are variations within the trunk (with the height and the distance between the medulla and the bark), being greater, generally, at the alburnum (from 80% to more than 200%) than within the heartwood(from approximately 40% to 100%). For the tree, water has a vital role, and its existence is indispensable. However, in wood, which is a hygroscopic material, the variation of the humidity ratio causes dimensional alterations. Its presence allows biological attacks, principally by fungae and insects, and impedes glueing or the finishing of manufactured products through the application of paints and varnishes. Thus, between living tree and the obtaining of the engineering material wood, a stage of removing water, or drying, becomes necessary.
The drying is the intermediate operation that most contributes to increase the value of the products manufactured from wood. However, it is one of the most costly stages in the transformation industry and, for this reason, there is a constant search for greater efficiency of the wood dryers and the actual drying process (JANKOWSKY, I. P. Improving the efficiency of dryers for sawn wood. Belém, 1999. A work presented at the IV International Plywood and Tropical Wood Congress, Belém, 1999. At print).
According to Ponce and Watai (see PONCE, R. H.; WATAI, L. T. Manual de secagem da madeira. [Manual for the drying of wood] São Paulo: IPT, 1985. 72p), the transformation of raw wood into products and consumer goods requires its prior drying for the following reasons: (i) it allows the reduction of dimensional movements to acceptable levels producing, in consequence, pieces of wood with more precise dimensions; (ii) it increases the resistance of the wood against fungi that cause stains and rotting and against the majority of xylophage insects; (iii) it improves the mechanical properties of wood, such as hardness, resistance to bending and compression; (iv) it increases the resistance of the splices and joints employing nails or screws; (v) it avoids the majority of flaws such as deformations, warping and splitting; (vi) it increases acoustic insulation properties and (vii) it facilitates the secondary beneficiation operations, such as turning, drilling and joining.
From the science and technology point of view, the concept of dry wood is a relative one, where a wood may be considered dry when its final humidity ratio is equal or less than the humidity equilibrium corresponding to its conditions of use (relative air temperature and humidity). This value will also depend on the type of product constructed from the wood and its use, as shown by Table 1 (Ponce and Watai, 1985).
TABLE 1Final humidity ratio recommended for certain woodproducts.ProductHumidity ratio (%)Commercial sawn wood16-20Wood for outdoor construction12-18Wood for indoor construction08-11Panels (plywoods, agglomerates,06-08laminates, etc.)Flooring and wainscotting06-11Indoor furniture06-10Outdoor furniture12-16Sporting equipment08-12Indoor toys06-10Outdoor toys10-15Electrical equipment05-08Packaging (crates)12-16Blocks for shoes06-09Firearm stocks and grips07-12Musical instruments05-08Agricultural implements12-16Boats12-18Aircraft06-10
The humidity ratio or quantity of water in the wood (U) is defined by the ratio between the mass of water present in the wood (ma) and the dry mass (ms). In this manner, it is possible to obtain the following expression:U=ma/ms
Where the total mass of the sample is represented by (mu), therefore:U=(mu−ms)/ms
Usually, the humidity of wood is expressed in terms of percentual, thus;U%=[(mu−ms)/ms]*100
By convention, the dry mass is obtained after the wood undergoes a drying in an oven at 105° C., until its stabilisation or constant weight.
Another very important parameter referring to humidity of wood is the Saturation Point of the Fibres (SPF) also known as the Cellular Wall Saturation Point. This is defined as the quantity of water necessary to saturate the cellular walls without leaving water free within the lumen. The humidity of the Saturation Point of the Fibres falls around 25 to 30%, depending on the plant species. Humidity above the SPF refers to the ratio of free water, also known as the green lumber stage, and, below the SPF refers to the hygroscopic or bonding water.
In consequence of alterations of the humidity below the SPF, dimensional variations of wood occur, meaning the contraction and expansion of the piece of wood, which occur due to the decrease or increase of the humidity, respectively.
This dimensional variation manifests itself in the three planar directions of the wood; the longitudinal, the radial and the transversal, which may be:                linear: that which develops along the three directions of the wood, having as unit of measure, the length (m) and        volumetric: expressed in volume (m3), resulting from the sum of the three variations.        
Possessing anisotropy (characteristic behaviour of wood), that is, different physical and mechanical properties on the longitudinal, radial and tangential plans of the tree trunks, the drying contractions are, generally, in the order of x in the radial direction, 0.1x in the longitudinal direction and 2x in the tangential direction. Thus, as the drying contractions are not equal in all directions, it is possible that there occurs a major change in the original shape of the piece, causing the appearance of deformations (warpages) and splits.
Considering the quality of the dried wood, the defects may be, according to Mendes and collaborators (1997) (MENDES, A. S.; MARTINS, V. A.; MARQUES, M. H. B. Programas de secagem para madeiras brasileiras. Brasília: IBMA 1997.114p):
Superficial Fissures: the superficial fissures appear when the traction stresses perpendicular to the fibres exceed the natural resistance of the wood, due to an excessively accelerated initial drying (high temperature and low relative humidity of the air). In these conditions, an excessive drying of the surface layers occurs, rapidly attaining low humidity values for the wood (inferior to the saturation point of the fibres), whilst the internal layers retain more than 30% humidity. This produces significant differences in the ratios of humidity between the surface and the centre of the wood (surface under traction and interior under compression), which may be aggravated by the anisotropy of the dimensional variations. The thicker the piece of wood, the greater the possibility of surface fissures occurring. These happen mainly in the initial phases of drying.
Splits at the Extremities or Ends: these are caused by the extremities drying faster when compared to the rest of the piece of wood. They occur, normally, at the beginning of drying.
Internal Fissures or Honeycombs: these appear during the drying, when the traction stresses develop in the interior of the piece (surface under compression and middle under traction) or reversal of the stresses. These stresses cause internal fissures when the efforts exceed the cohesion forces of the wood cells.
Superficial Hardening: during industrial drying there commonly occurs the development of compression stress at the surface and traction stress on the inside of the piece of wood, caused by the occurrence of a humidity gradient across the thickness. If these compression and traction forces are above the proportional limit (elastic limits) of the wood, residual deformations may occur that remain even when the humidity gradient across the thickness is eliminated.
Warping: this is any distortion of the piece of wood in relation to the original planes of its surfaces. Thus, taking into consideration the planes in relation to which alteration occurred, the warps may be half-pipe, longitudinal and twists. Although a large part of all deformations are frequently developed during drying, the control of the process and the conditions of drying are not always responsible for such deformations. This phenomenon may occur due to the innate properties of the wood, being inherent to its place of vegetative development. By definition of drying defects, such deformations are not part of the quality control, but rather are part of the quality of the wood. Mendes and collaborators, 1997, mention that, whilst little can be done to minimise the appearance of warps, it is possible to render the drying programs less severe (reducing the drying potential of each stage of the process), and also, very low final humidity ratios should be avoided, as the contraction of the wood increases with the decrease of the humidity ratio. In this sense, uniformity is important as it helps avoid that only a part of the load presents a ratio of humidity greatly below the desired level. Generally, the most efficient procedures to reduce warping are: adequate distribution, correct stacking with a perfect vertical alignment of the chocks, prior drying in open air before drying in the oven and restraint of the load by means of weight placed on top of the stack or traction of the stack with springs.
To minimise the prejudicial effects of the drying contractions of the wood in the quality of the final product it is necessary, before manufacturing the product, to reduce the initial humidity to a humidity corresponding to the surrounding conditions at the place of use.
However, the drying of the wood before the first transformation (production of planks, plys/laminates and chips) becomes impossible for the following reasons: (i) the geometric dimensions are inadequate (wood in logs) for undergoing controlled drying and are difficult to handle inside the drying equipment and (ii), due to the anisotropy, the differentiated contractions induce a series of deformations that are not compatible with the geometry, thus provoking the formation of fissures.
In practice, the drying of wood must be, therefore, undertaken after the first transformation and before all the further stages such as the beneficiation and the finishing.
The first attempts at drying wood date to the beginning of the 18th century with the use of the Cumberland method in which the wood was placed in the midst of wet sand to be curved and/or dried through the action of heat until attaining the suppleness and humidity desired.
At the end of the 19th century and the beginning of the 20th century industrial dryers already showed similar characteristics to those of today. Humid air began to be employed to control the drying speed of the wood producing, in this manner, a dry product of better quality.
The last and most important advance in the mechanical construction of wood dryers occurred in 1926 (see MILOTA, M. R. Drying wood: the past, present and future, In: INTERNATIONAL IUFRO WOOD DRYING CONFERENCE 6., 1999 Stellenbosch. Proceedings. University of Stellenbosch, 1999. P. 1-10, quoting Koehler, 1926), where the dryers began to have reversible air circulation and an automatic control device regulated by clock. As previously mentioned, the drying method presently employed is very similar to those of the 20's and 30's. The dryers have become larger allowing an increase in the volume of the dried wood per drying unit. The air heating pipes are now in the form of coils (radiators), the ventilators are placed in the upper part with the purpose of better ensuring air velocity, the humidity may be increased by the spraying of water or by the injection of saturated steam or, also, reduced through the partial renovation of the air within the drier or using the principle of condensing the air and, in the majority, computers are employed to control the process.
However, despite all the evolutions of recent years, if one were to chose a single plank randomly from a stack of dry wood, it would probably not be possible, with any degree of certainty, to know if it had been dried with the technology of the 30's or that of today. This does not mean that researchers have done nothing since 1930. Certainly, today, there is better knowledge of how water is distributed in wood and how it moves in it (Milota, 1999).
According to Krischer and Kroll (1956), quoted by Perré (1994), there are three distinct stages, from the physical aspect, during the drying process of wood, as follows:
First Stage: the drying speed is constant, thus, the evolution of the time for the loss of the mass of humidity in wood is linear. This phase commences after the stabilisation period of the thermal process and proceeds whilst the surface of the wood is irrigated with free water resulting from capillary action and the effect of internal gas pressure. During this phase, the speed of drying depends on the velocity and temperature conditions of the air, as well as the temperature of equilibrium of the wood with the humid air temperature.
Second Stage: it commences when the surface of the wood enters the hygroscopic phase. The speed of drying in this phase decreases. The temperature of the wood increases, starting at the surface, and approaches the dry air temperature.
Third Stage: theoretically, it commences when the wood is totally in hygroscopic phase. The speed of drying shows at this moment a new reduction, tending towards zero. The drying is completed when the temperature of the wood equals the dry temperature and the air humidity equals the equilibrium temperature of the wood (determined by the desorption isotherm).
Furthermore, many drying programs were developed and presented to the industrial sector in an attempt to improve the quality of the drying. These programs were created taking into consideration the differentiated behaviours of woods during drying, resulting from the heterogeneity of the physical, mechanical, chemical and anatomical characteristics of woods amongst species and even within the same species of tree.
The programs for industrial drying of wood may be of the following types: humidity-temperature, time-temperature, or based on the gradient for drying, also called the potential for drying (Rasmussen, 1968; Branhall & Wellwood, 1976 and Hidebrand, 1970, quoted by Galvão & Jankowsky, 1985 (see GALVÃO, A. P. M.; JANKOWSKY, I. P. Secagem racional da madeira. São Paulo. Nobel, 1985. 112p).
With the development of automation and the computerised control of the process, the humidity-temperature type programs came to the forefront of the wood drying industry sector, followed by those employing the gradient for drying, in accordance with Table 2.
TABLE 2Traditional program or table for drying used forPinus spp., aiming a final humidity ratio of 13% (Galvão &Jankowsky, 1985).Stage/RelativeEquilibrium(HumidityDry bulbWet bulbHygrometrichumidityhumidityof thetemperaturetemperaturedifferenceof the(UE)wood)(Ts)(Tu)(Ts-Tu)air (UR)(%)Heating60.0° C.59.0° C.1.0° C.95.0%20.6%>60%60.0° C.55.5° C.4.5° C.80.0%13.1%>60%/50%60.0° C.54.5° C.5.5° C.75.0%12.0%>60%/40%60.0° C.52.0° C.8.0° C.65.0%9.8%>60%/30%65.0° C.53.0° C.12.0° C. 55.0%7.7%>60%/20%75.0° C.57.5° C.17.5° C. 40.0%5.5%Uniformity75.0° C.69.0° C.6.0° C.76.0%11.0%Conditioning75.0° C.73.0° C.2.0° C.92.0%16.0%
Generally the programs are divided, systematically, into three stages:                stages of initial heating: this phase has the purpose of causing the heating of the drying chamber of the oven and the load of wood without allowing, however, the actual drying process to commence. High relative humidity is employed;        stages of actual drying: in this phase the removal of the humidity from the wood occurs. According to Galvão & Jankowsky (1985), low temperatures should be used during the removal of free water (40 to 60° C.) along with high relative humidity (85%). To avoid the occurrence of collapses in the species that dry with difficulty it is advisable to use a relative humidity above 85% and an initial temperature of around 30° C. It is also suggested that around ⅓ of the initial humidity should be taken as reference for commencing the reduction of the relative humidity. The temperature of the dry thermometer should be maintained constant until all the free water has been removed from the wood. The maximum values depend on the species and the thickness of the wood, thus, for greater thickness lower temperatures should be adopted. For humidity below 30% the dry temperatures may be considerably raised. The time period of this phase will depend on the density of the wood, the thickness of the piece, the temperature used and the humidity gradient, and        the stages of uniformity and conditioning: the uniformity phase may be dispensed with depending basically on the quality of the drying. But, the principal purpose is the uniformity of the humidity that occurs between the pieces of the load of wood. In the final stages of drying, the possibility of obtaining a humidity ratio that is similar for all the pieces is remote. The aim of the conditioning phase is the elimination of the internal stresses, Basically, this operation consists of significantly raising the relative humidity of the air in a manner as to cause a new humidification of the surface layers of the pieces, making the humidity gradient less abrupt or, also, increasing the temperature (up to approximately 100° C.) to release the stress gradients caused by drying.        
The patent U.S. Pat. No. 3,939,573 describes a drying process for wood at low temperature. The drying of the wood consists the following two stages: (i) employing an air temperature of around 20 to 30° C. until a humidity percentage varying between 16 and 25% is obtained, and (ii) raising the temperature to around 34 to 38° C. and maintaining it thus until obtaining the desired humidity ratio of the wood. This process takes as principle the use of drying temperatures similar to those normally encountered in natural conditions (on average 30° C). In this manner it is hoped that the mechanical resistance of the wood is not compromised. On the other hand, due to the low temperatures used, this process presents a long drying time and there are frequent occurrences of defects such as end splits. Furthermore, the woods submitted to this treatment, due to the surrounding conditions of the drying (high humidity and average temperature of 30° C.), are subject to attack by the fungi that cause stains.
As an example of thermal treatment at high temperature it is possible to quote the patent document WO 94/27102. This describes a drying process for wood consisting of the following stages: (1) thermal treatment at a temperature of at least 90° C., preferentially at least 100° C., and maintaining this temperature until the humidity ratio of the wood attains levels below 15% and (2) an increase of the temperature to values above 150° C. (preferentially between 180 and 250° C.) until the weight variation of the treated product attains around 3% at least. In this process, the use of high temperatures demands constant control of the temperatures on the surface and inside the wood, thus, maintaining the difference between these temperatures at around 10 to 30° C. If these conditions are not respected, the wood will present a series of defects such as fissures and warps.
The U.S. Pat. No. 5,992,043 patent proposes a thermal treatment for wood, with the aim of increasing the biological resistance and reducing the hygroscopicity. This process has three stages, illustrated graphically as zones “A”, “B” and “C”. The first stage, corresponding to zone “A” is a conventional drying stage where the temperature of the oven is progressively increased to about 80° C. The intermediate stage, corresponding to zone “B”, is a stabilisation treatment where the temperature is raised from the drying temperature of 80° C. to the glass transition temperature of dried wood which, in the present case, is the average between the temperature of lignin and of hemicellulose and which, according to literature, is normally above 150° C. It is mentioned that in this zone “B” (in the diagram, between 120 minutes and the td), the only object is the dimensional stability of the wood. The last stage, corresponding to zone “C”, also called drying or curing stage (curing treatment), consists in raising the temperature of the wood to around 230° C.
In this process, however, due to the use of an approximate value for the glass transition temperature—thus the average between the glass transition temperature of lignin and of hemicellulose in stage “B”—it is not possible to guarantee when treating more problematic woods (such as Quercus rubro and Eucaliptus spp.) the mechanical qualities of the material. Furthermore, during “C”, an elevation of the temperature occurs to values above that of glass transition, in this case approximately 230° C., to complete the thermal treatment of the wood, a process also know as roasting.
The woods resulting from such processes are intended for different uses than those woods that undergo conventional industrial drying process. Generally, when drying wood, with the exception of some conifers of temperate climates, temperatures above 100° C. are not employed (see Mendes et al., 1988).
It is important to highlight that despite all efforts undertaken to search for more uniform programs that comply with the difficult compromise between duration of the drying, consumption of energy and quality of the final product, the industrial drying of wood still leaves much to be desired. To date, only the experience of the drier operators, through the use of their empirical knowledge, has allowed anything close to this difficult compromise. This problem is aggravated, mainly when considering woods known to be problematical, as in the cases of the woods Quercus rubro and Eucalyptus spp.
In this manner, more efficient and profitable processes are being sought, capable of guaranteeing the physical and mechanical qualities of wood and allowing the use of a drying process that is the same for all species.
Therefore, the importance of a refined process for the drying of wood based on the neutralisation of the growth stresses, as well as those due to drying, through the use of the rheological properties of wood, becomes evident. This is the objective of the present invention.